Rotax struggling above 10,000 rpm

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  • #43964
    jack jensen
    Participant

    I was at SBR a week or so ago. The kart runs great up to 10k or 10,500 rpm then falls apart, sputtering and popping. I jetted richer and leaner and set the power valve from 27 turns out to 14 turns out. Jetting is 12/86. Tried a new plug with no difference. The only change since it was out last and running fine is removal of the factory airbox and a K&N filter added, only for clearance issues with new wide seat. Any suggestions would be greatly appreciated. Thanks, Jack

    #65982
    Doug Welch
    Participant

    Put the stock air box back on. The motor needs the back pressure created by it.

    #65983
    Rodney Ebersole
    Participant

    You posted 12/86 as your jetting, I assume that is your gear ratio? I am curious as to what was your largest main jet that you tried? Pooping is generally a lean condition that happens before a lean stick or failure. The K&N filter is a radical change in airflow to the motor which I haven’t tried. Just changing to a big main jet may not be enough change for correct fuel/air ratio.
    Many other settings like the float level and weight, the needle and slide position and sizes, emulsion tube, even the expansion pipe and fuel pump delivery could come into play in order to get the fine balance of air/fuel ratio it had with the stock air box and other stock parts.
    A crude test would be with about a 170 main jet then flip the choke on if it still poops to see if it runs a bit better. But your at risk, as with even a stock set up I have had just my float drop set wrong that caused it to run too lean, (bowl empty’ed) no matter how big of main jet was in it.
    Good Luck

    #65984
    Greg Welch
    Participant

    You really should put the stock box back on, and you can bend the bracket out to help it clear the seat if you need to. The airbox boot is also angled, so you can rotate it to allow you to bend the bracket out farther to clear the seat, and don’t be afraid to trim a little material out of the lip of the seat, but that is the last thing to do.

    The filter is pushing more air into an over carbureted engine than it can handle. We played with one for a while with no success. You would need to richen pilots, needle, floats and main to start, and It would probably need a different atomizer to run well.

    #65985
    Eddy Wyatt
    Participant

    Concur with Doug and Greg, if all you changed was the air box before having problems go back to your last good run set-up, i.e. get the air box back on. If all else fails suggest getting it to Greg. I ran Rotax in RMAX and TAG configurations 2006-2008 and had great success with Shockwave built and tuned Rotax’s. Recommend Doug and or Greg, they are amoung the best in the country when it comes to great Rotax performance and reliability.

    Respectfully,
    Eddy
    😀

    #65986
    jack jensen
    Participant

    Thanks for all the input. I will make the old airbox fit and I heard the newer style airbox is a little smaller (narrower) I will look for one of those too. I suspected the filter since it ran well before that change. I appreciate all of your advice. Thanks, Jack

    #65987
    Doug Welch
    Participant

    The “new” style air box filter is exactly the same shape as the old style. The difference is the height location of the filter portion. The “new” style fits in the chassis differently and allows more clearance. That said, you can bend the air box bracket and rotate the boot to angle the air box towards the right side of the kart to give more room for the seat. I have been able to get a XXXL seat in the kart and still get the air box to clear.

    #65988
    jack jensen
    Participant

    Awesome. Thanks again for the advice Doug. Jack

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